1. Field of the Invention
This invention concerns a semiconductor device. More particularly, this invention concerns a semiconductor device having a protection circuit.
2. Description of the Prior Art
Conventionally, a protection circuit has been used to protect a semiconductor device from a so-called electrostatic breakdown, namely from a breakdown due to a large voltage signal at an input terminal or an output terminal.
FIG. 1 shows a circuit diagram of a conventional protection circuit utilized in a semiconductor device. In the drawing, numeral 1 designates an input terminal. The input terminal 1 is connected to a following internal circuit. Numeral 2 designates a protection device which can be expressed as a bipolar transistor or a serial connection of two P-N junctions 11 and 12 in reversed direction. Hereafter, the protection device 2 is explained as a bipolar transistor. The collector electrode of the transistor 2 is connected to the input terminal 1, and the emitter electrode thereof is supplied with a reference voltage Vss. The base electrode of the transistor 2 is supplied with a potential Vbb of the semiconductor substrate of the semiconductor device. In the case where a P-type substrate is used, the Vbb is -2 to -3 volt, for example.
An electrostatic breakdown test is performed by applying a predetermined large voltage signal to the input terminal 1 with the supply of 0 volt as a reference voltage to the Vss terminal. In this protection circuit, when a large voltage is supplied to the input terminal 1 and the voltage between the input terminal 1 and the reference voltage terminal Vss exceeds the breakdown voltage of the P-N junction 11, the large voltage is bypassed through the bipolar transistor 2. Thus, the internal circuit can be protected from the electrostatic breakdown. The large voltage may be applied to the input terminal 1 not only during the electrostatic breakdown test but also due to the unexpected contact of a human with the semiconductor device, which may result in a discharge of static electricity.
Recently, the protection of the device from electrostatic breakdown is one of the most important factors in the reliability of the device. In the conventional semiconductor device, the protection circuit can protect the device from the electrostatic breakdown due to a large voltage between the input terminal 1 and the reference voltage terminal Vss. However, in the actual device, a large voltage may be supplied not only between the input terminal 1 and the reference voltage terminal Vss but also between the input terminal 1 and the power source voltage terminal Vdd or between the power source voltage terminal Vdd and the reference voltage terminal Vss.
Thus, it is desired that the device be protected from any electrostatic breakdown, such as that due to a large voltage between the input terminal 1 and the reference voltage terminal (Vss) (hereafter called first electrostatic breakdown), breakdown due to a large voltage between the input terminal 1 and the power source voltage terminal (Vdd) (hereafter called second electrostatic breakdown) and breakdown due to a large voltage between the reference voltage terminal Vss and the power source voltage terminal Vdd (hereafter called third electrostatic breakdown).